Coatings of Polyami 31. 31. RENFREW, HAROLD WITTCOFF, DON E. FLOYD, .LYD D. W. GL4SER Research Laboratories, General Mills, Inc., Minneapolis 13, M i n n .
T
HE epoxy (et.hoxy1ine) resins have had a considerable irnpact upon the coatings industry in the past decade (5). They are versatile materials which have been used in several different xays. One current method of application involves a twocontainer system in which the epoxy resin is mixed vc-ith a polyamine curing agent just prior to coating. The properties of the cured films, air-dried or baked. are highly attractive, but practical limitations have been imposed by the short pot life of t,he mixed paint and by adverse physiological activity of the volatile amine curing agents. This paper describes new types of blends containing epoxy resins which are useful as protective and decorative coatings. I n these new compositions the volatile anline curing agent has been replaced by nonvolatile polyamide resins of relatively lorn molecular weight, .rvhich contain reactive amine groups. Systems composed of mixtures of such polyamides and epoxy resins have somewhat, longer pot lives than the epoxy-amine systems, although the conditions of cure for the two systems are not greatly different. The resins offer greater handling ease from t,he point of view of minim1 health hazards. I n addition, the system based on polyamide resin-epoxy resin blends provide cost savings over the amine-cured systems. I n the paper the properties of the polyamide resin-epoxy resin system are described and comparisons are made a-ith the more conventional amine-cured systems.
developed to handle such tn-o-component systems, as the mixed epoxy resin and catalyst have a limited working life ranging fioin a f ' e ~minutrs to 1 da\-, depending upon the formulation POLYA.\IlL)E RESIR'S
Polyamide resins h v e been used in various ways as protective and decorative coatings. TT'ittcoff, Renfrew, and Speyer (13) have described gloss coatings for paper based on alcohol-hydrocarbon solutions of polyamide resin. Nordgren, Lichtman, and Champlin ( 7 ) have recommended polyamide resins as additi7.m t o shellac in order to improve mater resistance. Polyamide resins have been described by Murray, Liberti, and Allen ( 6 ) as intumescent' resinous coml)onents in flame-retardant paints. Polyamide resins' w e x-dl established as major components of adhesive compositions for cementing applications where paper such as label stock must adhere to other cellulosic materials, as well as to rubber, wood, glass, metal, and certain plastic surfaces. A recent patent ( 2 2 ) describes polyamide resins as essential components of an alkyd vhic>hin itself is thixotropic and n-hich ]\-ill contribute thixotropic properties v-hen milled with other .surface coating materials. The chemist'ry of the polyamide resins has been described in some of the above-mentioned publications as well as by Cowan, Lewis, and Fallcenberg (2). The resins are condensation golymers of dimerized (and trinierized) veget'able oil, unsaturated fatty acids, and aryl or alkyl polyamines. For use with epoxy resins, these polyamides are furt'her restrict,ed to those polyrners which contain a multiplicity of reactive centers, for conibination with the epoxide groups of the epoxy resins. These polpmide re,&s> of course, are related to the nylons hut are characterized by lox-er molecular weights. The difference in properties between the tn-o types of resins can be attributed not only to the lower molecular m i g h t of t,he polyamides, as opposed to the nylons, but also to the fact t,hat the carboxyl groups of thc polymer-forming acid are separated by M greatcr number of carbon atoms.
EPOXY RESINS
The application of epoxy resins in the protective coatings industry has been discussed by Johnson (6). These materials, which are formed by reaction of bisphenols with epichlorohydrin, range from viscous liquids to hard, brittle resins. il reprwentation of the epoxy resin structure is shown here.
0
r
POLYAMIDE RESIN-EPOXY RESIN BLEYDS
CH8 Paint manufacturers in the past have large13 used these iesins by esterifying the free hydrouyl groups with unbatnrated fatty acids to give a synthetic drying oil, or have cold-cut the resins in suitable solvents and mixed them n-ith modifiers Tvhich will react with the hydroxyl and epoxide groups at elevated temperatures to form baking finishes. (Urea-formaldehyde, melamine-formaldehyde, and certain phenol-formaldehyde resins have been used in this way to cross link the epoxy resins. Polybasic organic acids have been employed in similar fashion but less successfully in coatings systems.) A method which has been fast growing in importance, especially in formulating epoxy finishes that will cure a t room temperature, involves rapid-acting catalysts, such as organic sulfonic acids or organic amines (9). Compositions with ethylenediamine or diethylene triamine as the curing agents are now being widely used in coatings and adhesives. Special spray guns have been
The first report to a technical group on the polyamide-cposy resin blends was made by the NorthJTeetern Paint and T.'arnish Production Club in 1953 (8). These blends are also of inteycst for use as adhesives, cast,ings, electrical embedments, and glassreinforced plastics (3). The present paper, hovxver, is concerned chiefly with those properties of the epoxy-polyamide resin system Tvhich are of interest to the coatings chemist. This naturally includes coiisideration of adhesives properties, +ice the forces bonding paint) films t o the substrate are important. R7hen two materials separately form the basis for eficctive dhesives formulations, this does not guarantee that chemically reacted blends of the materials vi11 have good adhesion to a particular surface. The molecular configurations and intermolecular forces favoring the formation of strong bonds between the surface and the separate components may be modified by t.he rear:tion. Rut polyamide resins and epoxy resins, which can be separately formulated into widely useful adhesives, on mixing and curing do bond strongly to such diverse substrates as paper,
2226
October 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
wood, metals, glass, and some plastics. This observation, like most of the other phenomenological behavior of adhesives systems, is not easily explained on theoretical grounds. A qualitative description may, however, be helpful. Staverman (11) has pointed out that the adherence of an adhesive to a given material is determined not only by the capillary constants-e.g., the contact angle of the adhesive and the surface measured when the contact surface is decreasing-but also by what happens during the hardening process. One of the factors of pTimary importance is the shrinkage on hardening, strong adhesion being retained if the shrinkage is small. The influence of shrinkage will be diminished if the cured adhesive has a high elasticity. In the case of the polyamide resin-epoxy resin coating compositions, a system exists which is coated a t a relatively high solids concentration of about 40%; hence the volume does not contract excessively during the evaporation of the solvent. I n the same vein, the curing of epoxy resins with amine catalysts is a reaction characterized by a low volume change. I n the polyamide-epoxy resin system the formation of the three-dimensional, thermoset gtructure involves chiefly the reaction of amine groups and epoxy rings along the polymer chains. This entails the low shrinkage reaction and does not superimpose a large stress upon the adhesive bonds. Although the cured epoxy-polyamide resin blends &re not truly rubbery materials, they possess sufficient flexibility ant] &asticity in the normal case to accommodate the strains caused by solvent evaporation and reaction. At least, the coatings exhibit excellent adhesion. The cured materials also have a high cohesive strength, which accounts for their great toughness and good abrasion resistance. The reaction between polyamide resins and epoxy resins may be describcd as an interaction of two polymers to produce crosslinked structures. When the polyamide-epoxy resin blends are made with a high melting epoxy resin, thin films will dry rapidly to the tack-free stage. I n some measure this simply involves the evaporation of solvent; but, as the solids concentration of the creases, the rate of the cross-linking reaction also accelBecause the size of the reacted polyamide-epoxy molep l r : 4qFTFases very rapidly with only a few cross linkages formed, $he cqqtiqg is soon a t a stage \!*here it can be handled, although the purjng will continue for several days a t room temperature. puch coatings are slower in reaching maximum hardness than are epoxy compositions which are cured with amine catalysts of low molecular weight--e.g., ethylenediamine-but in general they ~$9be safely handled sooner despite this fact. Sctually, as is & p ~ y c in s9b;sequent tables, the polyamide-epoxy resin compositions require baking a t elevated temperatures in order to achieve maximum adhesion.. The hardness of air-dried and baked films, tiowever, is very similar. The reaction between amine groups and epoxy rings is exothermic. I n certain of the amine-catalyzed epoxy casting compositions this “exotherm” is sufficiently large to generate excessively high temperatures, leading to the objectionable volatilization of unreacted amines. The combination of reactive polyamide resins and epoxy resins cures effectively without such a large release of thermal energy. In setting up the network structure which characterizes the polyamide-epoxy system, it is presumed that fewer epoxy rings are being opened. In part this is simply a dilution effect, as it is customary to blend polyamide-epoxy resins in the ratio of 50 to 50 (or lower), whereas the common amine-catalyzed epoxy composition employs only 5 to 10% of organic amine with 95 t o 90% of epoxy resin. A rate factor is also involved. However, in forming the crosslinked structure with two polymeric constituents, fewer new Linkages are needed. Certainly the number of new chemical bonds formed in the polyamide-epoxy resin reaction are adequate to provide coat’ings with good solvent and chemical resistance (see Table 111). The insolubilization proceeds a t a rate
2227
which permits recoating in time cycles that are compatible with standard painting practice. EXPERIMENTAL
Typical properties of the reactive polyamide resins used in this study (General Mills Polyamide Resins 100 and 115)are tabulated below. Polyamide Resin 100 10-11 0 . 1 % max. 43 min.
Polyamide Resin 115 10-11 Ash, 7% by weight 0 . 1 % max Ball and ring softening point Viscous (ASTM), O C. liquid Viscosity a t 25O C.“ (Gardner-Holdt) A-C As-& Specific gravity a t % C. io 0.98 0.99 a For a 35% solution in a 1 t o 1 mixture of butanol and toluene a t 25O C.
Colora (Gardner 1933)
The General Mills polyamide resins are soluble in certain alcohols, chlorinated hydrocarbons, organic acids, and a few other solvents. Most useful are mixed solvents, composed of a combination of an alcohol higher than ethyl alcohol or an etheralcohol and a hydrocarbon. Mixtures such as isopropyl alcohol and toluene (or an aliphatic naphtha), butanol and xylene, and Cellosolve and xylene are effective combinations. Epoxy resins are available from several manufacturers. Many of the data in this paper were obtained with the Eponliresins (Shell Chemical Gorp.) (10).
PROPERTIES OF EPON RESINS Epon 864 Melting point, ’ C., Durran’s mercury method 40-45 Viscosity a t 25O C.” -4i-B Specific gravity, 20°/40 C. 1.188 Epoxy value, eq./100 g. b y pyridinium chloride method 0.29 Hydroxyl value,. eq./100 g. ,by lithium aluminum hydride method 0.26 Esterification value, eq./100 g. a For 40% solution in butyl carbitol.
...
Epon 1001
Epori 1004
Epon 1007
64-76 C-G 1.204
97-103
R-T
Y-Zl
0.19-0.20
0.11-0.1 2
0.05
1.154
127-133 1.146
0.32
0.34
0.36
0.74
0.55
0.50
The epoxy resins are soluble in ketones, esters, ether-alcohols, and chlorinated hydrocarbons. Mixed solvents, such as combinations of ketones and aromatic hydrocarbons, are especially effective. FACTORS IN FORMULATIOX OF POLYAMIDE RESIS-EPOXY RESIN COATINQS.Because there are several types of epoxy resins and two types of polyamide resins for use in these formulations, consideration must be given to which of the several combinations provide the best results in the most economical manner From the data described below, it may be concluded that blends of Polyamide Resin 100 and Epon Resin 1001 or its equivalent provide highly satisfactory coatings when blended in a ratio of 1 to 1. This combination also is the most economical. Polyamide Resin 115 may be substituted for Polyamide Resin 100 in formulations where its greater reactivity is of value. Cornbinations based on Polyamide Resin 115 have better chemical and solvent resistance, though somewhat poorer acid resistance. The optimum ratio here appears to be 65 parts of Epon Resin 1001 and 35 parts of Polyamide Resin 115. The choice of solvents is extremely important. For protective coatings for wood and metal, the polyamide resin may be dissolved in a mixture of butanol and xylene (1 t o 4). The epoxy resin may be dissolved in a mixture of methyl isobutyl ketone and xylene (1 to 1). The polyamide resin or the epoxy resin may aleo be dissolved in a mixture of Cellosolve and xylene (I to 9), although this solvent combination did not come to light until much of the work recorded in this paper had been accomplished. For paper coatings where application is by varnishing machine, Polyamide Resin 100 may be dissolved in isopropyl alcohol-toluene. The epoxy resin or the epoxy resin-Vinylite VliGH resin (see below) combination useful in paper coatings
2228
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 10
300" F. ( n o r m a l b a k i n g temperature for metal finishes) 400 F. (for quick cure)
Flexible Pol?amide Resin 100-Epon 1001 Coatings on .4luminum Foil and Collapsible Tubes
~-
Breaking of metal occurs before paint film is ruptured in c r u i n d i n n tests
is best diss.olved in a mixture of methyl ethyl ketone, toluene, and isopropyl alcohol. For thinning, however, higher boiling solvents are preferred. For rotogravure coating, the polyamide resin may be dissolved in isopropyl alcohol-met'hyl ethyl ketone, whereas the eposy resin-Vinylite resin combination may he dissolved in methyl ethyl ketone alone. This is desirable, as rotogravure application requires extremely fast drying. Brushing vehicles may employ, as solvents, Solvesso 150 (St,andard Oil Co.)-Cellosolve (9 to 1) for the polyamide resin and Solvesso 150-Cellosolve (1 bo 1) for the epoxy resin. The problem of pigmentation is an important one. I n general, polyamide resin-epoxy resin combinations are compatible ii-ith all common pigments. Grinding either by ball mill 01'13: rollcr mill produces fine grinds and uniform dispersions. Pigments may be ground by means of a ball or pebble mill in the less viscous epoxy resin solution. However, the polj-amide solution may well be used in roller mill grinding, n-here high viscosity is deMirahle and where the inherently good Ivetting action of the polyamide resin may be employed to advanmge. The question of modifiers in addition to the two primary constituents already discussed is an important one. llodifiern n-hich were found useful in the course of this ~ o r l includr: r Vinylite S'AGH (Bakelite Division. Linion C:irbide and Carbon), useful for accelerating drying of paper cocttiiigs. DC-200 ( D o a Corning Co.), useful :is a 91i:i agent for paper coatings. Some other si!iconrs appear t o lead t o crxtcring problems. MDAC (7-methyl-4-dicthj-!arninocou!n.irin) iCarlislc Chemical.Co. j, an optical xhiteiiing agent used in paper coat,iiig formulas. Tenlo-70 (Griffin Chemical Co. ), an antifloating agent usc3i'ul in enamels cont,aining tvio or more different pigments. Lecithin, an agent which improvrs the speed of grinding of pigments and facilitates vietting. Formic acid, useful in extending the pot lives of solutions of mixtures of Polyamide Resin 100 and epoxy resins. I n air-dry coatings it tends to lower the resistance to water and alkali and Impact, but it has no adverse effects in baked coatings. Beetle 216-8 (American Cyanamid), a leveling agent, helpful ln overcoming the tendency toward cratering, somet,imes noticed in films of cured epoxy resins. Basic coating characteristics were determined by studying rate of cure for blends (at different ratios) of various epoxy resins with Polyamide Resins 100 and 115 (General Mills) a t four different temperatures. The epoxy resins selected for this study were Epon Resins 864, 1001, and 1007 (Shell Chemical Corp.). The temperatures chosen for study were: Room temperature, 7 5 " F . (air dry) 150" F. (for wood finishes)
Itate of curc was followed by nieasuring hardness developed at different time intcwals. F o r t h e u n b a k e d films (room temperature cure) flexibility and impact resistance were also measured, since, s u r p r i s i n g l y enough, these valucs were found to improve as the systenu lvere allowed to stand. I n actual practice it has been observed that films tend to yellov if halring ovens are not clean. Gas fumes are particu l a ~ l j , dvletciious. In clean ovens with good ventilation no difficulties have been experienced.
PREPARATION OF COA'1'1SG S Y S T E l l S
The preparation of the coating systems ia extremely simple, requiring only that the tvio resins IIC dissolved separat'ely a t solids concentrations of 50 to TOY0. In this work the polyamide portion of the I'ornda under test, was dissolved at 50% solids in a mixture of isopropyl alcohol and toluenc. Other systems containing an alcohol higher than ethyl alcohol iri :tddition t o an aromatic hydrocarbon would have served equall- well. Ordinarily> equal ratios of the alcohol arid hydrocarbon xere used, although for the sake of economy, the solvent mixture may comprix a mixture of alcohol and hydiocarhon in the ratio of 30 t o 70. Thr epoxj- resin in this 1 to 1 mixt,ure of mcthyl is If Yinylite VAGH TTas formulatioris, it Tvas dissolved with the epoxy resin, using a solvent comLina*ioii of methyl ethJ-l ketonp, toluenc, and isopropyl alcohol. Alternatively the epoxy resin may be dissolved in toluene-methyl ethyl ketone and the 1-iiiylite rcsin in methyl ethyl ketone alone, after which the tn-o may be mixed. The P fornier procedure, hoivever, is the ~ I ~ O T roiivenient. For pigmented coatings, either the epoxy resin solurion or the pol!-amide resin solution, or both, may be pigmented. I n the a-ork described here the epoxy resin solution was pigmented and sufficient pigment v-as used t o achievc the proper pigment volume concentra.tion, once t'he pigmented epoxy solution was mixed with the unpigment'ed polyamide resin solutioii. Pigmentation may be carried out by standard procedures on a roller mill or with a ball mill. Films were applied by a 1.5-mil Bird applicator to clean panels and cured either at, room temperature or a t an elevated temperature as indicated. The substrates consisted of glass, tinplated steel, cold-rolled steel, or glassinc paper, depending on the nature of the test to be applied. TEST JZETHODS
The cured films viere subjected to thc tcsts tabulated below. The baked films were allowed to cool to room temperature prior to testing. Air-dried films were allowed to dry for 1 week prior to testing, unlces otherwise indicated. I n most tests, comparisons were made with amine-cured epoxy systems. Comparisons beyond this are not included, as it is believed that the aliphatic amine-cured eposy systems are sufficiently well established in the coatings industry t o provide a point of reference.
October 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
HARDNESS.Sward rockei method ( 4 , page 164). Average values are reported. FLEXIBILITY. Mandrel procedure (4, page 188). IMPACT R E S I S T ACE. ~ Gardner impact tester (4,page 188A). Both 31-gage tin plate and 22-gage steel plates were used. For steel plates the tube was extended to allow a maximum impact of 172 inchpounds. ABRASIONRESISTANCE.The Taber abraser u a s used as described in ASThI standards ( 1 ) . ALKALI RESISTANCE(4, page 515) Five per cent sodium hydroxide was used, evcept as noted in tables. The tubes were immersed in individual compartments, which were covered to prevent evaporation and weakening of the alkali by carbonation. ACID RESISTANCE.Measured by spot tests on films with 20% acetic or 50% sulfuric acid. MOISTURE VAPORTRA described in TAPPI Standard T428m-40. GREASEPROOFNESS. As described in TAPPI Standard T454m-44.
2229
Resistance of Baked Films on Steel to Alkali, Acid, and Impact 241-10. Silicone-modified alkyd enamel 241-9. Alkyd-melamine enamel Polyamide Resin 100-Epon 1001 enamel 241-2. Upper section of panels. 20% NaOH, 24 hours Middle section. 50% H*SOa, 24 hours Lower section. 172 inch-pound impact
Many of the above tests are somewhat subjective in nature, because of uncontrollable variables such as operator judgment and the inability to produce films which time and time again have precisely the same surface properties. Subjective factors are commonly taken into consideration in all evaluations of paint films.
blends of Polyamide 100 with Epons 864, 1001, and 1007, cure is complete within 6 minutes at 400" F., 10 to 20 minutes a t 300" F., and 1 to 2 hours a t 150" F. At room temperature the coatings are tack-free and capable of being haiidlrd mithin a few hours. Homever, maximum hardness and flexibility are not developed for several days. R 4 T E OF CURE With epoxy resins of high molecular weight, cure is rapid and Test results for rate of cure with different compositions a t diffilms are hard, while with epoxy resins of lower molecular weight, ferent curing temperatures are shown in Tables I and 11. rure is slower and films are softer. Epon 1001 or an equivalent C O N C L U S I O ~For S . most of the coating formulas based on resin is the generally preferred choice for use with Polyamide Resins 100 and 115 in coatings. Higher ratios of epoxy resin to polyamide resin lead to TABLE I. CURINGRATE harder films, and convrrsely, Rocker Hardness Rocker Ilai dness Rocker IIardness higher ratios of polyamide a t 150' F_a t 300' F a t 400' F GO 120 180 10 20 7 6 9 resin to epoxj resin lead to Composition Min Min hlin hlin Min \Iin hlin Alin s o f t e r , m o r e flexible films. Epon 1001-Polyamide 100 Rate of cure does not seem to 40 GO 35 46 46 25 34 45 50 52 50.50 55 57 58 47 56 44 61 Ti0 be changed seriously by modest 60 40 53 61 60 GG 62 45 62 70 change in ratio. Epon 1007-Po1)aniide 100 40 60 50.50 60 40 Epon 864-Polyamide 100 40 GO 50 50 60 40 Epon 1001-Poljaniide 115 50.50 65 35 Epon 1001-6% diethylenetriamine
43 59 64
43 59 68
43
39 61 53
44 57 49
45 GO 68
55
67
56 GO 72
24 28 20
38 37 32
40 45 34
24 48 42
24 41 49
14
46 54 55
48
54 64
59 58
65 58
50 55
70
75
74
G8
58
TABLE 11. CURINGRATEAT ROOMTEMPERATURE
Composition Epon 1001-Polyamide 100 40 60 50 50 Epon 864-Polyamide 100 50 50 Epon 1001-Polyamide 115 BO 50 65 351001-670 ethylene Epon
diamine a
Tack-Flee to Foil, hlin
Rocker Hardness 1 7 D a y Days
100 65
20 33
290 120 100
14 48 39
110
28
34 57 40
Impact Resistancea 1 7 Day Days
